This invention relates to a method for characterizing the adhesion of hard materials to one another, and in particular to a method for physically stressing the materials in the region of the interface between the materials to aid in such characterization.
In recent years, considerable research has been devoted to the application of hard, thin coatings to hard materials by chemical or physical vapor deposition processes. Such coatings are applied to enhance the chemical and physical properties of the substrate materials. The goal of such research is to optimize the coating/substrate design to achieve maximum reduction of factors inducing failure of the substrate in use. Typical of such factors are abrasive wear, chemical wear, and thermal degradation. Examples of such optimized coating/substrate design are described in U.S. Pat. Nos. 4,965,140, 4,950,588, and 4,988,564, all commonly assigned herewith and incorporated herein by reference. Further examples are described in U.S. Pat. No. 4,892,792 (entitled "AlN Coated Silicon Nitride-Based Cutting Tools"), also commonly assigned herewith and incorporated herein by reference.
In order to facilitate optimization of the properties of a coating/substrate composite article, the characteristics of potential substrate and coating materials and their interaction must be understood, particularly their behavior in the service environment. Based on such understanding, maximally compatible materials may be selected for a given use. Many of such properties are known, or may be calculated or speculated and empirically verified. Although the interfacial strength, or adhesion (adherence), of materials in such composite articles is a critical factor in the selection of compatible materials, this factor has remained difficult to quantify and is poorly understood. Adhesion has been defined as the amount of energy required to separate a coating from its substrate.
Adhesion is a macroscopic property influenced by many factors, including interdiffusion of materials across the interface between the substrate and the coating, compound formation at the interface, coating and substrate morphologies, defect structures, and residual stresses. Such residual stresses are due to deposition procedures or to differences in material properties, e.g. thermal mismatch between the substrate and the coating. Absolute determination of these factors, their interdependence, and the resulting influence on the mechanical properties of the interfacial region is the subject of ongoing research. Without adequate quantification of these parameters, accurate theoretical prediction of coating adherence is impossible. Presently, determination of adherence must be approached empirically through mechanical testing.
Various techniques have been used in attempts to quantify the adherence between coatings and substrates, for example the well known peel test, pull test, indentation test, scratch test (using the Revetest.RTM. apparatus), and thermal shock-laser test, as well as many others. These tests are only partly successful at best, and apply to only a narrow field of materials.
The scratch test is the most widely used adherence test for hard, thin coatings. In this test, a statically loaded, generally conical, diamond tipped indenter is drawn across the surface of the coating as the load is increased, until coating failure is induced. Since the force of the load in this test is applied in a direction normal to the coating surface, the coating, substrate, and interface all are increasingly deformed as the load is increased. Thus when failure occurs, it is difficult to determine the exact point, i.e. in the coating, within the substrate, or at the interface, at which the failure was initiated. Because of the degree of deformation of the coating, substrate, and interface, this prior art scratch test has proved to be less a measure of the adherence of the coating than of the durability of the coated material.
It clearly would be desirable, in order to promote further scientific development and engineering improvements of coated material systems, to have available a technique which is more sensitive to the pure adhesive and cohesive forces between various coatings and substrates. The present invention addresses this need.